Dynamic porous coating for orthopedic implant

ABSTRACT

A dynamic porous coating for an orthopedic implant, wherein the dynamic porous coating is adapted to apply an expansive force against adjacent bone so as to fill gaps between the dynamic porous coating and adjacent bone and to create an interference fit between the orthopedic implant and the adjacent bone.

REFERENCE TO PENDING PRIOR PATENT APPLICATIONS

This patent application:

(i) is a continuation-in-part of pending prior U.S. patent applicationSer. No. 13/764,188, filed Feb. 11, 2013 by Matthew Fonte et al. forPOROUS COATING FOR ORTHOPEDIC IMPLANT UTILIZING POROUS, SHAPE MEMORYMATERIALS (Attorney's Docket No. FONTE-15), which patent applicationclaims benefit of prior U.S. Provisional Patent Application Ser. No.61/596,900, filed Feb. 9, 2012 by Matthew Fonte et al. for POROUS, SHAPEMEMORY MATERIAL, ORTHOPEDIC IMPLANT COATING (Attorney's Docket No.FONTE-15 PROV);

(ii) claims benefit of pending prior U.S. Provisional Patent ApplicationSer. No. 61/612,496, filed Mar. 19, 2012 by Matthew Fonte et al. forPOROUS, SHAPE MEMORY MATERIAL, ORTHOPEDIC IMPLANT COATING (Attorney'sDocket No. FONTE-17 PROV);

(iii) claims benefit of pending prior U.S. Provisional PatentApplication Ser. No. 61/661,086, filed Jun. 18, 2012 by Matthew Fonte etal. for “DYNAMIC” ORTHOPEDIC COATINGS MADE OF SPACER FABRIC (Attorney'sDocket No. FONTE-18 PROV); and

(iv) claims benefit of pending prior U.S. Provisional Patent ApplicationSer. No. 61/738,574, filed Dec. 18, 2012 by Matthew Fonte et al. forPOROUS, SHAPE MEMORY MATERIAL, ORTHOPEDIC IMPLANT COATING (Attorney'sDocket No. FONTE-24 PROV).

The five (5) above-identified patent applications are herebyincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to medical apparatus and procedures in general,and more particularly to porous coatings for orthopedic implants.

BACKGROUND OF THE INVENTION

Femoral stems with reduced stiffness have been introduced in total hiparthroplasty to facilitate proximal load transfer and thereby reducestress shielding and periprosthetic bone loss. However, poor implantfixation and unacceptably high revision rates are a major problem withthese prostheses. One reason for this is that the implant is preciselymachined and the femoral canal is frequently not, leaving gaps as largeas 0.025″ between the implant and the wall of the femoral canal. In manyinstances the implants may only have 35% of their surface area in directcontact with the adjacent bone. See FIG. 1. This lack of a tight fitbetween the implant and the surrounding bone is a significant problem,inasmuch as a tight fit is required between the implant and the adjacentbone in order to provide maximum fixation in the shortest time, bymaximizing implant stability and the opportunity for bone ingrowth.

For successful implants, sufficiently regenerated bone fills the gapbetween the implant and the host bone, so that the implant is firmlyattached to the surrounding bone.

To overcome problems with implant loosening, implants need to stimulaterapid bone regeneration in order to replenish the missing bone and/or tofix the implant firmly within the host bone. To succeed as an orthopedicimplant, the implant must provide a habitat for bone-forming cells(e.g., osteoblasts) so that the bone-forming cells can colonize on theimplant surface and synthesize new bone tissue. Frequently the implantsare not compatible with the bone cells responsible for bone formation,and instead promote the formation of undesirable fibrous soft tissue.Such fibrous soft tissue does not adequately support the implant, whichleads to implant loosening under physiological loading conditions andeventual implant failure. Thus, in order to design more successfulorthopedic implants, one needs to take into account the cellularprocesses that promote bone ingrowth. Positive responses fromosteoblasts, including increased initial adhesion, proliferation anddifferentiation (from noncalcium-depositing cells to calcium-depositingcells) are essential. Coordinating activities between osteoblasts andthe bone-resorbing cells (e.g., osteoclasts) is also needed in order toprovide healthy bone around the implant. Poor communication betweencells can lead to bone necrosis adjacent to the implant, thereby causingloosening of the implant. Another undesirable occurrence is theformation of fibrous soft tissue by fibroblasts. Excessive fibrous softtissue formation hinders osteoblast/osteoclast activities and hencelimits bone regeneration. Due to these cellular events, the orthopedicfield has concentrated on understanding cellular recognition of surfacesand creating biomaterial surface properties which maximize suchinteractions for the creation of more bone and enhancedosseointegration.

One way to improve the performance of bone implants is to modify thesurface texture of the implants. Many studies have shown thatmicrostructural features such as grain and particle size promoteosteoblast functions better than smooth surfaces. This motivates the useof nanophase materials for orthopedic implants.

Macrostructural features such as porous coatings are another means forimproving osseointegration of the implant. Today, hip implant stems aretypically a composite structure consisting of a substrate (typicallyformed out of a cobalt chrome alloy or a titanium alloy) which carriesthe patient's weight, and a porous surface coating mounted on theimplant substrate. This porous surface coating (which is generallyreferred to in the industry as a “porous coating”) comprises peaks andvalleys, whereby to aid in immediate implant fixation and ultimatelypromote long term stability through osseointegration of the host bonewith the porous coating. See FIG. 2.

Prior to inserting the implant, the surgeon broaches the femoral canalto create a cavity that, ideally, closely matches the geometry of theimplant (which is then inserted into the cavity in the bone). However,this fit is not always perfect, and gaps frequently exist between theimplant and the bone. These gaps cause the implant to “point load” thesurrounding bone, and also create barriers which inhibit rapid andeffective osseointegration of the implant.

Today, the majority of porous coatings are made of titanium or tantalum.These porous coatings are “static”, in the sense that they aresubstantially rigid. These porous coatings are textured, and are appliedto the implant substrate by hot plasma spray, chemical and/or physicalvapor deposition, by chemically etching thin films and plates, and/or bysintering and/or diffusion bonding metal beads or metal fibers into asolid rigid mass. See FIG. 3.

In addition to producing a substantially rigid structure, the coatingprocesses used to produce porous coatings tend to produce a largelytwo-dimensional structure for the bone to grow around. There is no meansfor the bone to tunnel further into the porous coating so as toestablish significant three-dimensional osseointegration. Thus, thelargely two-dimensional porous coating may stifle or compromiseeffective long-term osseointegration of the implant due to the lack ofsignificant three-dimensional osseointegration. Additionally, thelargely two-dimensional porous coating structures created using theseprior art technologies do not accurately mimic the structure oftrabecular (i.e., cancellous) bone, which is three-dimensional andincludes interconnecting networks of pores with capillarity properties.See FIG. 4.

Recently, there have been advances in the creation of porous coatingsthat more accurately resemble trabecular bone. These porous coatingshave interconnecting networks of pores which are similar to those oftrabecular bone, and may serve to promote bone ingrowth deeper into theporous coating and hence provide better long-term implant fixation. Onemethod known in the art for creating such a porous coating is throughthe replication of an open cell network. In this method, a structuresimilar to trabecular bone (e.g., a polyurethane foam) is coated withanother material (e.g., titanium or tantalum) by vapor deposition, lowtemperature arc vapor deposition (LTAVD), chemical vapor deposition, ionbeam assisted deposition and/or sputtering. The underlying structure(e.g., the polyurethane foam) may then undergo pyrolysis so as to removethe underlying structure (e.g., the polyurethane foam), leaving ametallic structure which can be attached to the hip implant substrate(e.g., by sintering, brazing, diffusion bonding, gluing or cementing,etc.). See FIG. 5. However, the porous coating produced by this methodis static, i.e., it is substantially rigid.

Other methods for forming a porous coating include chemical vapordeposition of commercially pure tantalum onto a porous carbon scaffoldand then sintering the resulting structure onto the substrate of theimplant. See FIGS. 6 and 7. Again, however, the porous coating producedby this method is static, i.e., it is substantially rigid.

Depending on the desired thickness of the struts of the porous coating,the physical or chemical vapor deposition process may be sufficient toreproduce the scaffold structure; however, it is also possible to morerapidly thicken the struts following deposition through the bulkapplication of a powdered metal or granulates made of titanium, tantalumor other biomaterials, with or without a binder (e.g., methylcellulose).Following the application of the powdered metal, the scaffold issintered to integrate the powder or granulates. See FIG. 8. In any case,however, the porous coating produced by this method is static, i.e., itis substantially rigid.

SUMMARY OF THE INVENTION

The primary purpose of the present invention is to improve theperformance of implants by creating an implant with a dynamic porouscoating that changes shape, either through expansion or lateralmovement, so as to fill a gap with adjacent bone and to apply pressureagainst adjacent bone. The dynamic porous coating is flexible, andexpands to apply controlled pressure against the adjacent bone, promotesbone remodeling, improves fixation through faster osseointegration andreduces the gaps between the implant/bone interface.

The dynamic porous coating of the present invention preferably comprisesshape memory materials which are less stiff than titanium 6al-4v andcobalt-chrome alloys. These shape memory materials can include Nitinol,near beta or fully beta titanium alloys, shape memory polymers (e.g.,thermoplastic block copolymers) and biodegradable shape-memory polymersystems, all of which can be processed to be either superelastic or tohave shape recovery characteristics. These dynamic porous coatings havea 2D or 3D porous structure which can be multi-layered, and may coverthe implant either partially or entirely. The pores of the dynamicporous coatings can be infiltrated with a mixture of hydroxyapatite,tricalcium phosphate and/or other bone-making agents known in the art soas to further promote osseointegration. The dynamic porous coatings ofthe present invention can be made primarily of shape memory materialsand augmented with non-shape memory materials to help control strength,stiffness, biocompatibility and pore size for optimal osseointegrationproperties. The present invention finds utility as a dynamic porouscoating in a wide range of orthopedic implants where fixation andosseointegration are desirable, e.g., hip implants, knee implants,shoulder implants, elbow implants, spinal implants, extremity implants,dental implants, cranial and maxillofacial implants, etc. (sometimeshereinafter referred to simply as “implants”).

Significantly, the dynamic, space-filling, porous coating of the presentinvention has the ability to fill the gaps which frequently occur whenseating the implant in a broached bone hole, and thus more evenly loadthe surrounding bone and increase the capacity for fast and effectiveosseointegration. As a result, the dynamic, space-filling, porouscoatings of the present invention have particular utility in revisionsurgery (e.g., in hip revision surgery) where, after a prior prosthesishas been removed from bone, the bone cavity may be irregular and henceproblematic gaps may be more likely to occur around the replacementimplant.

In one preferred form of the present invention, there is provided adynamic porous coating for an orthopedic implant, wherein the dynamicporous coating is adapted to apply an expansive force against adjacentbone so as to fill gaps between the dynamic porous coating and adjacentbone and to create an interference fit between the orthopedic implantand the adjacent bone.

In another preferred form of the present invention, there is provided anorthopedic implant comprising a substrate and a dynamic porous coatingsecured to the substrate, wherein the dynamic porous coating is adaptedto apply an expansive force against adjacent bone so as to fill gapsbetween the dynamic porous coating and adjacent bone and to create aninterference fit between the orthopedic implant and the adjacent bone.

In another preferred form of the present invention, there is provided amethod for providing therapy to a patient, the method comprising:

providing an orthopedic implant comprising a substrate and a dynamicporous coating secured to the substrate, wherein the dynamic porouscoating is adapted to apply an expansive force against adjacent bone soas to fill gaps between the dynamic porous coating and adjacent bone andto create an interference fit between the orthopedic implant and theadjacent bone;

inserting the orthopedic implant into a bone cavity in the patient sothat the dynamic porous coating applies an outward force againstadjacent bone so as to fill gaps between the dynamic porous coating andadjacent bone and to create an interference fit between the orthopedicimplant and the adjacent bone.

In another preferred form of the present invention, there is provided adynamic porous coating for an orthopedic implant, the dynamic porouscoating comprising:

a spacer fabric comprising:

-   -   an inner layer formed by fibers;    -   an outer layer formed by fibers;    -   the outer layer being spaced from the inner layer; and    -   the outer layer being connected to the inner layer by a        plurality of connecting fibers extending between the inner layer        and the outer layer, such that the outer layer is capable of        applying an outward force against adjacent bone.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention will bemore fully disclosed or rendered obvious by the following detaileddescription of the preferred embodiments of the invention, which is tobe considered together with the accompanying drawings wherein likenumbers refer to like parts, and further wherein:

FIG. 1 is a schematic view showing gaps between an implant surface andbone;

FIGS. 2 and 3 are schematic views showing static porous coatings of theprior art;

FIG. 4 is a schematic view showing the structure of trabecular bone;

FIGS. 5-7 are schematic views showing prior art processes for forming astatic porous coating;

FIG. 8 is a schematic view showing processes for thickening a staticporous coating;

FIG. 9 is a schematic view showing a dynamic porous coating formed inaccordance with the present invention;

FIGS. 10-14 are schematic views showing various porous coatings formedin accordance with the present invention;

FIGS. 15 and 16 are schematic views showing various forms of knitting;

FIGS. 17-19 are schematic views showing dynamic porous coatings formedby knitting;

FIGS. 20 and 20A-20G are schematic views showing a dynamic porouscoating formed out of spacer fabric;

FIGS. 21-23 are schematic views showing various weaving techniques;

FIG. 24 is a schematic view showing how several wires may be twistedinto a single helical wire;

FIGS. 25-29 are schematic views showing fabrication of a Woven BulkKagome (WBK) weave;

FIGS. 30-33 are schematic views showing how corrugation can be used toform a dynamic porous coating;

FIGS. 34-43 are schematic views showing how honeycomb- and truss-basedstructures can be used to form a dynamic porous coating;

FIGS. 44-46 are schematic views showing how sintered beads can be usedto form a dynamic porous coating;

FIGS. 47-49 are schematic views showing how lamination of multiplelayers can be used to form a dynamic porous coating;

FIG. 50 is a schematic view showing how surface texturing may be appliedto the dynamic porous coating;

FIG. 51 is a schematic view showing the micro- and nano-structure ofbone;

FIG. 52 is a schematic view showing the material properties of corticaland cancellous bone;

FIG. 53 is stress-strain diagram for bone,

-   Nitinol and stainless steel;

FIG. 54 is a schematic view showing the austenite, martensite anddeformed martensite phases for shape memory alloys;

FIG. 55 is a schematic view showing the Nitinol phase diagram;

FIG. 56 is a schematic view showing the time-temperature-transformation(TTT) diagram for Nitinol;

FIG. 57 is a schematic view showing how a shape memory material dynamicporous coating can exhibit superelasticity or shape memory effect;

FIG. 58 is a schematic view showing use of a superelastic porous coatingin bone;

FIG. 59 is a schematic view showing superelasticity with ahoneycomb-shape structure;

FIG. 60 is a schematic view showing how a dynamic porous coating cancomprise an inner porous coating and an outer porous coating, whereinthe inner porous coating is different than the outer porous coating;

FIG. 61 is a schematic view showing the pore structure of a dynamicporous coating; and

FIGS. 62-65 are schematic views providing further details of porouscoatings formed in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As noted above, prior art porous coatings are static (i.e., rigid)structures, fixed in shape and geometry, which limits their ability toinfiltrate the bone tissue. As such, bone tissue must infiltrate intothe porous coating in order to achieve some level of osseointegration.With prior art porous coatings, initial osseointegration is limited tothose regions where the porous coating is in direct contact with thebone surface. Initial osseointegration is not possible where a gapexists between the implant and the bone, which is frequently the case inpractice.

In contrast, the present invention provides a porous coating which isdynamic in nature, preferably made out of shape memory materials (SMM),e.g., Nitinol (NiTi) per ASTM F2063, Ti-13Nb-13Zr per ASTM F1713,Ti-12Mo-6Zr-2Fe (TMZF) per ASTM F1813, etc. The dynamic porous coatingof the present invention expands against, and applies strain against,the host bone so as to stimulate bone remodeling and expedite/enhanceosseointegration. The dynamic porous coating of the present invention isdesigned to be “space filling” in nature, increasing the surface area ofthe implant that is in direct contact with the bone surface. Thus, thedynamic porous coating of the present invention applies bone-buildingstrain against the bone tissue and, in the process, also infiltratesinto the bone tissue, thereby providing true three-dimensionalosseointegration.

Biomedical applications are intended to be the main applications for thedynamic porous coatings of the present invention. The dynamic porouscoatings are preferably formed out of shape memory materials (SMM). SMMporous coatings offer the following advantageous properties: (i) goodbiocompatibility; (ii) a combination of high strength (which isimportant to prevent deformation or fracture), relatively low stiffness(which is useful to minimize stress shielding effects) and hightoughness (which is essential to avoid brittle failure); (iii) a porousscaffold for bony ingrowth; and (iv) shape-recovery behaviorfacilitating implant insertion and ensuring good mechanical contact withthe host bone, whereby to provide mechanical stability and enhancedosseointegration.

The dynamic porous coating of the present invention, which is preferablymade of a shape memory material, can be produced by any appropriatemethod, however, it is presently preferred that the porous coating beproduced using one of five methods:

(i) replication of a porous trabecular analog structure by coating foam(or a carbon foam) structure;

(ii) wire knitting, weaving, or braiding;

(iii) honeycomb- or truss-based structures;

(iv) sintered beads; and

(v) lamination of multiple layers, as will hereinafter be discussed infurther detail.

The dynamic porous coating of the present invention can be produced as asheet or shell that can be wrapped or placed over an implant substrateand then secured to the implant substrate, either by mechanicallyattaching it to the implant substrate, dynamically contracting it toadhere to the implant substrate and/or by metallurgically attaching itto the implant substrate (e.g., by braising, welding, sintering,diffusion bonding, hot isostatic pressing, etc.).

The SMM porous coating, which is essentially an exoskeleton structurefor the implant substrate, can be superelastic (SE), which is capable ofrestoring its shape once it is unconstrained and made to spring back;and/or it can have shape memory effect (SME) which allows it to bedynamic under the influence of temperature change, e.g., warming from acolder temperature to body temperature, or cooling from a warmertemperature to body temperature. The SMM dynamic porous coating can be“squished” flat and either superelastically, or through SME (temperaturechange), spring outward once the implant is inserted into the femoralcanal (or other bone cavity) so as to apply pressure on the bone tissueand lock the implant in place. See FIG. 9. The dynamic porous coating ofthe present invention can be applied to implants made of conventionalmaterials such as cobalt-chrome alloys and titanium alloys; or thedynamic porous coating can be applied to implants which are themselvesmade of a shape memory material such as Nitinol.

Replication of a Porous Trabecular Analog Structure

Deposition of shape memory material onto a polyurethane trabecularanalog results in a dynamic porous coating having the basic dodecahedronstructure of trabecular bone. See FIGS. 10-12.

Additionally, the open pore structure of the polyurethane foam can firstbe reproduced using a slurry of hydroxyapatite (or any other knownbone-forming compound), with or without a binder (e.g., methylcelluloseor other appropriate binder). This hydroxyapatite slurry is used to coatthe surface of the polyurethane foam scaffold (without filling thepores), then it is dried, heated to burn off the polyurethane foamscaffold, and finally sintered. This results in an open cellhydroxyapatite structure which is similar in shape to the initialpolyurethane foam. This hydroxyapatite structure can then be used as thetrabecular bone analog for creating the shape memory material porouscoating, i.e., the hydroxyapatite structure is coated with a shapememory material. By way of example but not limitation, thehydroxyapatite structure can be sputtered with Nitinol, whereby tocreate a superelastic metal dynamic porous coating with a hydroxyapatitecore. Additional processing to the shape memory material/hydroxyapatitescaffold (e.g., providing abrasion to the outer surface) can causeregions of the shape memory material to be selectively removed, therebyexposing the hydroxyapatite core. This exposed hydroxyapatite core isthen available to osteoblasts so as to aid in osseointegration of theimplant over a prolonged period of time. As the hydroxyapatite isconsumed/remodeled, it will leave behind an internal cavity for the boneto grow into. See FIG. 13.

It is also possible to create a comparable structure by using shapememory material powder to either replicate or coat the trabecular analogstructure (created from polyurethane foam) instead of using a physicalor chemical deposition method. By way of example but not limitation, itis possible to create a porous Nitinol coating using a slurry of eitherpowdered or granulated Nitinol with or without an appropriate binder(e.g., methylcellulose). This shape memory material slurry can bedeposited on the surface of the trabecular structure (eitherpolyurethane foam or a hydroxyapatite structure created from thepolyurethane foam). The shape memory material slurry is then allowed todry, whereafter it is sintered to fuse the powdered or granulatedparticles of Nitinol into an elastic porous structure (i.e., the dynamicporous coating). Alternatively, a slurry of powdered titanium and nickel(in the appropriate proportions) can be deposited on the surface of thetrabecular analog scaffold, and then sintered at the appropriatetemperature so as to form shape memory Nitinol (and hence to form adynamic porous coating).

Porous Nitinol structures (i.e., dynamic porous coatings) can also bemade by utilizing the so-called self-propagating high-temperaturesynthesis (SHS) reaction process. In this process, titanium powder ismixed with nickel powder and then compacted so as to form a compact.Energy (typically in the form of a plasma torch or hot filament) is thenapplied to the compact. Sufficient energy is supplied to the compact tocause the titanium and nickel to form Nitinol. It will be appreciatedthat when the reaction is complete, a porous Nitinol scaffold (i.e., adynamic porous coating) has been created. See FIG. 14.

Wire Knitting, Weaving, And Braiding

It is also possible to create dynamic porous structures (i.e., dynamicporous coatings) from shape memory wire. More particularly, many shapememory materials are readily available as wires in a variety ofdiameters. By way of example but not limitation, some commerciallyavailable NiTi and Ti 13Zr-13Nb wires range from 0.0004″ to 0.025″thick. Flexibility and overall diameter of the shape memory wire can beincreased by braiding or twisting together multiple wires of smallerdiameters so as to create larger structures. Additionally, bioabsorbablefilaments can be used in conjunction with shape memory filaments tocreate a dynamic porous coating that can remodel over time, therebyproviding a structure with increased initial strength, and having theability to resorb as new bone grows within the pores of the dynamicporous coating. In addition to filaments and wires, tubes can be usedfor forming the dynamic porous coating. The tubes may have a pluralityof radial holes to facilitate bony ingrowth. If desired, these tubes canbe filled hydroxyapatite or other bone-forming substances so as to aidin osseointegration.

Knit structures, with their high number of individual fibers, allow forthe creation of very intricate porous patterns, and hence provide theopportunity for increased performance capabilities for the dynamicporous coating. Warp and weft are two different types of knitting whichmay be used to form a dynamic porous coating. Warp knits have fibersthat extend along the length of the material, while weft knits havefibers that extend across the width of the material. See FIGS. 15 and16. Knits can be created in either sheets or tubes. Multiple sheets canbe laminated on top of one another, and tubes can be formed concentricto one another, in order to achieve a more three-dimensional structurefor the dynamic porous coating. In addition, by laminating sheets ortubes with different porosities, a more complex overall pore structurecan be created for the dynamic porous coating. See FIGS. 17-19.

In one preferred form of the invention, shape memory material fibers areknit into a highly porous, highly elastic “spacer fabric” which thenserves as the dynamic porous coating. More particularly, and looking nowat FIGS. 20 and 20A-20D, the shape memory material spacer fabriccomprises two opposing faces which are separated from one another, butwhich are also interconnected to one another with filaments which extendbetween the two faces and fill the region therebetween, providing anelastic cushion between the two faces. The two opposing faces of thespacer fabric can have the same or different mesh sizes, with surfacepores of various geometries including, but not limited to, honeycombsand rhomboids. The length of the wire filaments extending between thetwo faces (i.e., the connecting filaments which fill the space betweenthe two faces) can be varied as desired so as to adjust the thicknessand spring factor of the overall spacer fabric material. The shapememory material spacer fabric can be formed as a sheet or strip that canbe wrapped around the implant, or as a sleeve that can be slid over theimplant, whereby to form the desired dynamic porous coating for theimplant. The shape memory material spacer fabric can then be sintered tothe implant stem prior to aging and shape-setting.

Among other things, the shape memory material spacer fabric of thepresent invention provides a dynamic porous coating for an implant thatexpands and applies controlled, chronic pressure against the bone,promotes long term bone remodeling, improves secondary fixation throughfaster osseointegration and reduces the gaps between the implant/boneinterface which can inhibit proper osseointegration. In one preferredform of the invention, the new dynamic porous coating is formed out of aspacer fabric comprising Nitinol fibers (i.e., Nitinol wires) which arewoven in an appropriate manner so as to form the overall spacer fabricconstruct. In this form of the invention, the SMM spacer fabricpreferably comprises two separate fabric faces which are knittedindependently of one another and then connected by filaments whichextend between the two separate fabric faces and fill the spacetherebetween, whereby to provide the spring factor of the overall spacerfabric. See FIG. 20A. These nonwoven fabrics can be produced on bothcircular and flat knitting machines. They may be produced as a flatsheet, or as a cylindrical tube.

Spacer fabrics have three distinct layers. See FIGS. 20A, 20B and 20C.When used as a porous coating for an implant, the three layers of thespacer fabric have three different functions, i.e., the top layerprovides for bone ingrowth, the middle (vertical) fiber layer providesfor elasticity, and the bottom layer provides for brazing to theimplant. The top layer of the spacer fabric can have a structure ofrepeating dodecahedrons, i.e., a honeycomb geometry similar to that ofcancellous bone, whereby to facilitate osseointegration. The bottomlayer of the spacer fabric can be engineered so as to facilitatebraising to the implant. The middle (vertical) connecting layer of thespacer fabric, which comprises the fibers which extend between the toplayer of the spacer fabric and the bottom layer of the spacer fabric andprovide spring, is continuous and is knit vertical (e.g., at an angle of30°-150° from the top and bottom layers. See FIG. 20C. Thus, the spacerfabric is three-dimensional in construction, and it is the verticalfibers that create the elastic response for the spacer fabric when thespacer fabric is compressed, and/or made to bend, and then allowed torecover. The three ply structure of the spacer fabric has goodbreathability, wettability, crush resistance, and a 3D porousappearance, which makes it ideal for use as a dynamic porous coating.Each layer of the spacer fabric can be made of different materials andhave different porosity levels and geometry. These spacer fabrics can bestacked one on top of another to form a multi-level spacer fabricconstruct.

The spacer fabric can be designed to have an overall porosity rangingfrom 45% to 98%, with pore sizes ranging from 100-600 microns(0.004-0.02 inch), with an average of 300 microns (0.01 inch), wherebyto facilitate its use as a dynamic porous coating. See FIG. 20D. Themodulus of elasticity of the spacer fabric can be engineered so as tohave a modulus of elasticity of between 0.1 GPa and 5 GPa, with adesired modulus of elasticity approaching 1.6 GPa (230 ksi), which canbe highly beneficial when the spacer fabric is used to form a dynamicporous coating. Significantly, due to the inherent nature of a spacerfabric, the modulus of elasticity of the spacer fabric can varyaccording to the degree of compression imposed on the spacer fabric,i.e., when the spacer fabric has relatively little compression imposedon it, it will have a relatively low modulus of elasticity, whereas whenthe spacer fabric has a large compression imposed on it, it will have arelatively high modulus of elasticity. The surface roughness can begenerated from either the basic knit structure of the SMM wires crossingover each other, or can be enhanced with a plasma sprayed surface to anaverage 2500 Ra, which is much coarser than that of conventional porousmaterials. The plasma spray can be used to stiffen the construct andcontrol pore size.

A “scratch-fit” between the dynamic porous coating and the cortical boneduring implant insertion is desirable. This scratch-fit action causesthe rough surface of the implant to scrape the walls of the femoralcanal, filling the small pores of the dynamic porous coating with boneand providing excellent initial stability. The spacer fabric can beplasma sprayed for increased surface roughness.

In one preferred form of the invention, the dynamic porous coating isfabricated out of shape memory materials so as to provide a porouscoating of maximum elasticity. However, it should also be appreciatedthat the aforementioned three-layer spacer fabrics are deliberatelydesigned to be elastic, i.e., the material can be compressed and, uponremoving the compressive force, return to its original geometry. Thus,dynamic spacer fabrics suitable for the present invention can bemanufactured using a wide variety of metallic and non-metallic fibers,including non-shape memory material fibers, provided that the fibers arebiocompatible and can provide the requisite spring factor for the spacerfabric.

Thus, the dynamic porous coating of the present invention may comprise aspacer fabric formed out of non-shape memory materials such as stainlesssteel, cobalt chrome alloys, titanium, tantalum, niobium, zirconiumalloys, etc.

In one preferred form of the invention, the dynamic porous coatingcomprises a spacer fabric formed out of shape memory materials such asbinary and ternary Nitinol (nickel-titanium). Such shape memorymaterials have been shown to be biocompatible metals, and are routinelyused for medical implants. These materials can be drawn into wire, andused as the starting material for the spacer fabric. Among other things,the following shape memory materials may be used to form the spacerfabric used to form the dynamic porous coating.

Alloy ASTM Elastic Ultimate Tensile Designation Standard Modulus (GPa)Strength (MPa) Ti-6Al-4V ELl F 136 98 860 Ti-6Al-4V F 1472 110 930Ti-6Al-7Nb F 1295 99 900 Ti-15Mo F 2066 77 690 Ti-13Nb-13Zr F 1713 64550 Ti-12Mo-6Zr-2Fe F 1813 74 931

Alternatively, the dynamic porous coating may comprise a spacer fabricmanufactured out of biocompatible polymers, including shape memorypolymers. These polymers may include one or more of the following:

-   -   Polyethylene Terephthalate (PET)    -   Polypropylene (PP)    -   Polyetheretherketones (PEEK)    -   High-performance polyethylenes (UHMWPE)    -   Bioabsorbable polymers        -   Polyglycolic acid (PGA)        -   Poly-L-lactide (PLLA)        -   Polycaprolactone (PCL)        -   Various copolymers    -   Shape memory polymers        -   Polyurethanes        -   Block copolymer of polyethylene terephthalate (PET) and            polyethyleneoxide (PEO)        -   Block copolymers containing polystyrene and            poly(1,4-butadiene)        -   Triblock copolymer made from poly(2-methyl-2-oxazoline) and            polytetrahydrofuran        -   Polyetheretherketones (PEEK)

The diameter of the starting fiber greatly determines the mechanicalproperties of the final spacer fabric structure. Thicker fibers resultin a stiffer final construct. Preferably, the diameter of the fiber isbetween about 0.01 inch and 0.0002 inch. Most preferably, the fiber isbetween about 0.007 inch and 0.003 inch.

As noted above, the spacer fabric has three distinct layers. These threedistinct layers can be manufactured using three distinct wire sizes. Asan example, it is possible to use a large wire size for the base of thematerial so as to increase the surface area available for bonding to theimplant, a medium wire size for the filler material (i.e., theconnecting fibers) in order to give the spacer fabric appropriatestiffness and elasticity, and a fine wire size for the top surface(i.e., the surface contacting the bone) so as to better match thecancellous bone structure.

As noted above, while spacer fabrics manufactured from conventionalmaterials are dynamic in nature, and hence may be used to form theporous coating of the present invention, the dynamic effect of thespacer fabric can be enhanced by using a shape memory material (SMM),e.g., Nitinol (NiTi) per ASTM F2063, to form the spacer fabric (andhence to form the dynamic porous coating). The SMM porous coating can besuperelastic, (SE) which allows it to restore its shape once it isunconstrained, and/or it can have shape memory effect (SME) which allowsit to be dynamic under the influence of temperature change, i.e., bodytemperature. As an example of an SME application, the dynamic porouscoating of the present invention can be in the compressed state at atemperature below body temperature (37° C.), and following implantationto the medullary canal, warm to body temperature and return to itsoriginal uncompressed shape. This will fill any voids between theimplant and the bone, and where constrained, apply chronic bone-buildingstrain to the bone.

FIG. 20E shows exemplary orthopedic implants (e.g., the stem of afemoral hip implant, the cup of an acetabular hip implant, and thefemoral component of a knee implant), with a dynamic porous coatingformed out of shape memory material spacer fabric.

FIG. 20F shows the dynamic porous coating filling the gap between thestem of a femoral hip implant and the surrounding bone.

FIG. 20G shows a dynamic porous coating formed out of Nitinol,superelastic wire, with the dynamic porous coating having a mean poresize of 243 μm and with the dynamic porous coating being approximately92.5% porous.

A dynamic porous coating formed out of shape memory material spacerfabric provides a significant improvement over conventional staticporous coatings. By way of example but not limitation, the followingchart shows how a dynamic porous coating formed out of shape memorymaterial spacer fabric provides superior bone ingrowth (in both depthand area percent) over conventional static porous coatings:

2 WEEK RABBIT STUDY DATA Bone Bone Ingrowth Ingrowth (depth) (area %)dynamic porous coating formed 1700 μm 18 ± 6% out of shape memorymaterial spacer fabric Ti Plasma Spray 433 μm 8 ± 6%

By way of further example but not limitation, the following chart showshow a dynamic porous coating formed out of shape memory material spacerfabric provides superior holding strength over conventional staticporous coatings:

It is also possible to create porous coatings out of shape memorymaterials (SMM) using weaving techniques. There are several differenttypes of weaving techniques. They include: plain weave, twill weave,plain dutch weave, and twill dutch weave. See FIG. 21.

Weaving shape memory materials enables the creation of dynamic porouscoatings having a wide variety of different geometries and structures.See FIG. 22. It is possible to weave mono- and multi-layer dynamicporous coatings out of shape memory materials, as well as tubularstructures. It is possible to weave structures with varying widths andthicknesses within the same structure. Multiple layers of wovenstructures can be laminated one on top of another so as to create a morethree-dimensional structure. By layering multiple sheets of materialswith different pore sizes and geometries, one on top of another, adynamic three-dimensional fabric structure can be created which has acomplex interconnected network of pores. The dynamic porous coating canbe constructed as a single, or multilayered, sheet that can be wrappedaround the implant stem, or as a single or multilayered concentric tubethat can be slid over the implant stem. The shape memory material porouscoating can then be sintered to the implant prior to shape-setting.

It is also possible to weave a dynamic three dimensional SMM porousstructure directly. More particularly, using a jig, it is possible toweave wires and/or tubes in various patterns, building layers one on topof another, whereby to create an overall three dimensional porousstructure which can then be used as a dynamic porous coating for animplant. See FIG. 23. The overall three dimensional porous structure canthen be sintered to fuse the woven wires together if desired.Furthermore, if desired, a less porous, or non-porous, sheet can beattached to one or both of the faces of the woven three dimensionalstructure. This may be beneficial for the face that is to be sintered tothe implant stem, as it will have more surface area in contact with theimplant stem, and hence provide increased bonding of the dynamic porouscoating to the implant.

Another method for weaving a three dimensional shape memory material(SMM) porous structure is to use a modified Kagome weave. In thisapproach, the Wire Woven Bulk Kagome (WBK) is assembled from continuousSMM helical wires which are systematically arranged in six directions.The SMM helical wires may themselves be created from twisted SMM wire.See FIG. 24.

With WBK, the three dimensional structure is created in layers.According to the number of layers to be utilized, each Kagome plane isindividually assembled (see FIG. 25) and placed one on top of another ina fixing frame (see FIG. 26), with an appropriate space between eachlayer. The space provided between each layer helps determine the overallheight of the three dimensional structure.

With the Kagome weave, with the layers aligned on top of one another,three additional SMM helical wires are woven through the gaps of thethree different out-of-plane directions, creating the overall threedimensional wire-bulk Kagome. See FIG. 27.

The hexagonal openings in each layer of the WBK create a near-sphericalpore structure in the three dimensional weave. By changing the diameterof the starting wire and the pitch of the woven members, the pore sizeof the weave can be changed. Additionally, it is possible to create thisweave using tubes instead of wires, and the nodes of the weave can bebrazed with differing amounts of filler so as to alter the rigidity ofthe overall WBK structure. See FIG. 28.

When a compressive force is applied to the WBK structure, it readilycollapses, with the pores nesting as it collapses: one layer of porescollapses toward the right, and the adjacent layer collapses towards theleft. This allows for large amounts of deformation. See FIG. 29.

For the application of a dynamic porous coating for an implant, it ispossible to create the WBK structure using shape memory material wiresor tubes. This provides a WBK structure having superelastic properties,which is ideal for application as a dynamic porous coating for animplant. The SMM WBK can be created as a multilayered sheet and eitherwrapped around the implant, or cored so as to create a sleeve that canbe slid over the implant. The SMM WBK is then sintered onto the implantprior to shape-setting, whereby to provide a dynamic porous coating forthe implant.

Additionally, it is possible to corrugate any of the foregoingstructures to increase their porosity and spring factor. Corrugation canbe performed in one or two directions. The corrugation can beaccomplished by bending the layers at defined regions, or pressing thelayers between two dies. Additionally, the corrugation can be linear,resulting in pyramid-shaped structures, or wavier structures, resultingin an egg-crate structure. See FIG. 30. Individual layers may becorrugated prior to layering them one on top of another (see FIG. 31),or the overall structure itself can be corrugated (see FIG. 32) afterhaving already been formed by layering individual layers.

It also may be beneficial to secure the corrugated structure to aless-porous sheet in order to aid in sintering the dynamic porouscoating to the implant stem. See FIG. 33.

Honeycomb- and Truss-Based Structures

Another method for creating an open pore trabecular structure out of ashape memory material (whereby to form a dynamic porous coating) is tocreate honeycomb- or truss-based structures. See FIGS. 34 and 35. In oneform of the present invention, the honeycomb structure can be 3D, suchas a dodecahedron structure, similar to a “nano buckyballs” geometry,and is superelastic and/or has SME (shape memory effect via temperaturechange). See FIGS. 36 and 37. It may also be made up of a repeatingpattern of diamonds, hexagons, or other shapes.

There are many methods known in the art for creating honeycomb- andtruss-based structures. By way of example but not limitation, honeycombstructures can be created using a HOBE method (Honeycomb BeforeExpansion), a corrugation process, or a strip-slotting method. In theHOBE manufacturing process, multiple layers of material can be stackedon top of each other, welded along defined regions, sliced, and pulledapart so as to create the expanded honeycomb. See FIG. 38. In thecorrugation manufacturing process, a press can be used to corrugate asheet of material. Multiple sheets of material can then be stacked ontop of each other so as to create a honeycomb structure. Alternatively,the corrugated sheets can be layered perpendicular to one another,creating a cross-ply structure. See FIG. 39. In the strip-slottingmethod, strips of material are notched so that they can be interlockedso as to form the desired honeycomb structure. The interlocked stripscan then be welded or braised together for additional support. See FIG.40.

To increase the overall porosity of the honeycomb structure, which canbe desirable for its use as a dynamic porous coating, holes can beformed in the faces of the sheets, either prior to, or following, thecreation of the three-dimensional honeycomb structure. See FIG. 41.

Furthermore, by staggering the orientation of the hexagonal elements(instead of aligning them one on top of another), a more complex porestructure can be created for the dynamic porous coating. See FIG. 42.

Sheets of honeycomb can be layered one on top of another, and attachedto the outside surface of the implant stem by braising, sintering, orlaser welding, whereby to form a dynamic porous coating for the implant.See FIG. 43.

Sintered Beads

Dynamic porous coatings can also be fabricated by sintering togetherbeads. The beads can be spheres, or other shapes, so as to increase theoverall porosity of the dynamic porous coating. The beads can be laidout in individual layers, and then sintered together so as to fuse theminto sheets. The sheets can then be stacked one on top of another so asto build a multi-layered three-dimensional dynamic porous coating.Different porosities can be achieved by using beads of different shapes.As an example, using spherical beads of the same diameter results in afairly dense structure (see FIG. 44); however, using spherical beads oftwo different diameters generates a much more porous structure (see FIG.45).

Additionally, the beads need not be spherical. By way of example but notlimitation, the beads can be made in the shape of “jacks” (see FIG. 46)or another three-dimensional shape. By assembling the beads in themanner discussed above, it is possible to create even more porousstructures with increased interconnectivity between the pores, wherebyto facilitate their use as a dynamic porous coating for an implant.

Lamination of Multiple Layers

Another method for producing a dynamic porous coating is to laminatemultiple thin sheets of shape memory material one on top of another. Apattern can be chemically etched, punched, or cut out of each of thesheets and, by altering the geometry of the pattern on each sheet, it ispossible to create a porous dodecahedron or other multi-facet structurewhich can function as a dynamic porous coating for an implant. Moreparticularly, each sheet can be layered one on top of another andsintered together so as to create a dynamic porous structure. Bychanging the geometry of the cut-out on each layer, it is possible tocreate many different dynamic porous structures. The dynamic porousstructure can be made in a sheet and wrapped around the implant, or asleeve can be cut out of the material and the implant stem placed withinthe sleeve, whereby to form a dynamic porous coating for the implant. Byway of example but not limitation, four sheets of Nitinol (see FIG. 47),each with different geometries, can be stacked one on top of another(see FIG. 48) so as to create a dynamic porous coating having athree-dimensional structure.

Another method for manufacturing a dynamic porous coating is to use anadditive manufacturing technique. One such technique uses metal powder,with a particle size of from about 10 μm to about 200 μm (i.e., Nitinolpowder), and an energy source (e.g., a laser or electron beam) to buildstructures layer by layer, selectively sintering powder together so asto build a three dimensional shape. More particularly, a thin layer ofthe powder is spread out as a uniform layer, and then an energy sourceselectively melts regions of the powder, fusing the particles together.Another layer of powder is then spread on top of the first layer, andthe energy source again melts regions of the powder. This processcontinues until the complete three-dimensional dynamic porous coating isbuilt. See FIG. 49. Using this manufacturing technique, it is possibleto construct both solid and porous structures, or combinations of thetwo (e.g., a solid section for attachment to the substrate of an implantand a porous section for engagement with bone). It is possible tomanufacture sheets of the dynamic porous coating that can then bewrapped around the implant stem, sleeves of the dynamic porous coatingthat can be slid over the stem, whereby to create implants with dynamicporous coatings mounted to the stem.

Coatings

Regardless of the manufacturing process used to create the dynamicporous coating, it may be desirable to apply a coating of anothermaterial on top of the dynamic porous coating. By way of example but notlimitation, a Nitinol porous coating may be covered with a thin coat oftitanium or a bone agent such as HA (e.g., by sputtering or plasmaspraying). This titanium coating would seal the Nitinol and prevent theNitinol from being in direct communication with the body. Thus, if aconcern exists regarding nickel leaching out of the Nitinol and into thebody, this titanium coating would be one approach for protecting againstthis. The coating can be applied in a number of ways, such as, but notlimited to, physical vapor deposition, chemical vapor deposition, powdermetallurgy, and electroplating. Additionally, the plating process may beperformed so as to increase the opacity of the dynamic porous coating onX-ray or MRI. If desired, a titanium oxide layer can be formed on theNiTi so as to enhance biocompatibility.

Surface Texturing

Additionally, in order to aid in osseointegration, it may be beneficialto modify the surface of the dynamic porous coating so as to provide itwith additional nano-texturing. This may be accomplished by sinteringnon-uniform, non-spherical powder to the surface of the dynamic porouscoating. This powder may or may not be a shape memory material. Surfacetexturing may also be accomplished by using starting materials that havea rough finish. As an example, a knit dynamic porous coating may be madefrom wire that has been pickled or etched prior to knitting so as toincrease its surface roughness. Alternatively, etching or pickling maybe performed on the final dynamic porous coating prior to sintering tothe implant stem. See FIG. 50.

Implantation

While it is possible to insert an implant with a dynamic porous coatinginto the medullary canal of a bone by driving it in and forcing thedynamic porous coating to comply with the internal geometry of themedullary canal, it may also be beneficial to use a rapidly dissolvingpaste to keep the dynamic porous coating in the compressed state duringinsertion. As an example, calcium hydroxylapatite (HA)[Ca₁₀(OH)₂(PO₄)₆], β tricalcium phosphate (TCP) [Ca₃(PO₄)₆],fluorapatite, and biphasic calcium phosphate, a HA/β TCP combination,can be made into pastes, slurries, or cements that dissolve overcontrolled amounts of time.

An additional benefit to using one of these materials is that they areproven bone growth agents. HA is often plasma-sprayed onto a hip stem toincrease the rate of osseointegration. By using a rapidly dissolving HApaste to temporarily compress the dynamic porous coating, the implant isdelivering localized bone growth agents directly to the site where theyare needed.

As an example, a TCP paste can be applied to the dynamic porous coating.The dynamic porous coating can then be compressed and the TCP allowed todry. Once the TCP is dried, the compressive force can be removed, andthe TCP will hold the dynamic porous coating in the compressed state.The implant can then be inserted into the medullary canal. The nativephysiological solution at the site of implantation will dissolve the TCPcoating, and the dynamic porous coating will be allowed to expand,filling any gaps between the implant and bone, and where constrained,apply chronic bone-building strain to the bone.

Alternatively, sterile saline or another biocompatible solution can beirrigated to the site of the implant to aid in dissolving the TCPcoating.

Attachment of Dynamic Porous Coating to the Implant

Traditionally, porous coatings have been attached to the implantsubstrate by using a solid state diffusion or sintering process thatapproaches the melting temperature of the materials being joined.However, the elevated temperatures required for sintering havedeleterious effects on the underlying substrate and include reducednotch toughness and reduced fatigue properties. The elevated pressuresrecommended for sintering may also cause a decrease in overall porosityas the porous coating may partially collapse under the pressure andelevated temperature. Additionally, sintering will occur wherever twomaterials are in contact, thus unintentionally increasing the stiffnessof the post-sintered material and compromising fatigue strength at thesintered sites.

By way of example but not limitation, if a metal spacer fabric were tobe sintered onto a titanium stem substrate, not only would the bottomlayer of the metal spacer fabric sinter to the stem of the implant, butalso any place a fiber touched another fiber would be sintered together.This would greatly increase the stiffness of the spacer fabric,potentially rendering it non-elastic, i.e., static. For this reason,where the present invention comprises metal spacer fabrics, and/or wherethe components of the dynamic porous coating will be negativelyinfluenced by sintering, it may be preferred that the dynamic porouscoating be secured to the implant using a non-sintering technique.

To this end, instead of sintering the dynamic porous coating to thesubstrate of the implant, the dynamic porous coating may preferably bebrazed to the substrate of the implant. Brazing is a metal-joiningprocess whereby a filler metal is heated above its melting point anddistributed between two or more close-fitting parts by capillary action.The filler metal is brought to slightly above its melting (liquidus)temperature while protected by a suitable atmosphere, usually a flux, avacuum, or an inert atmosphere. It then flows over the base metal, knownas wetting, and is then cooled to join the pieces together. The dynamicporous coating which is brazed to the substrate preferably adheres tothe following ASTM specifications:

Test Spec Outcome Shear Strength F1044 Shear strength of the surface/substrate interface >20 MPa Shear Fatigue Strength F1160 Shear fatiguestrength of surface coating should be test to 10⁷ cycles. TensileStrength F1147 Tensile strength of the surface/ substrate interface >22MPa Abrasion Resistance F1978 Average mass if liberated porous coating<65 mg/100 cycles

For a medical application, the brazing material must be biocompatible.Silver (Ag) is one example of a biocompatible brazing agent. The brazingof Ag with Ti causes a eutectic transformation, creating TiAg (η) whichfacilitates the joining of Ti with the spacer fabric material withoutcausing any deleterious intermetallic phases to form. Normally, brazingoperations avoid eutectic reactions so as to not melt the surface of thematerial, which can create a deleterious, brittle recast layer. Brazingtitanium with silver occurs at approximately 1740° F. (950° C.), atemperature low enough to not cause a deleterious effect to the implantnotch strength or fatigue.

Since, during the brazing operation, the silver will wet and flow undercapillary action, it is important to use a uniformly thin layer ofsilver. If too much silver is used, it will melt and fill the pores ofthe dynamic porous coating. One method for applying a uniform thin layerof silver is to electroplate the silver onto the surface of the implantstem.

Methods of electroplating Ag onto a Ti stem are known in the art,however, they require a nickel or copper strike layer to first bedeposited on the implant. Ni and Cu are not biocompatible, and thereforecannot be used for the present application. For this application, wherethe Ag does not need to have strong adherence to the substrate, it ispossible to grit blast the surface of the implant, chemically etch it,and plate a layer of silver onto the titanium stem. While the Ag willnot be tightly bound to the substrate, it is bound sufficiently well toallow the dynamic porous coating to be pressed against it during brazingwithout the Ag flacking off. The thickness of the Ag preferably rangesfrom 100 nm to 5 μm. Another approach is to ion implant Ag to thetitanium substrate. Yet another approach is to create the dynamic porouscoating (e.g., SMM spacer fabric) with clad wire which has Ti or NiTi inits core, and Ag on its outer surface, and then braze the clad wire tothe implant, without a need to pre-plate the substrate prior to brazing.

The use of a silver brazing agent provides additional significantadvantages. More particularly, the rate of infection in primary totalhip arthroplasty is approximately 1-3%, and the rate of infection afterrevision of infected hip prostheses is up to 14%. While silver serves aprimary function as a brazing agent, it is also well known for itsbactericidal properties and the use of silver on medical devices haspreviously been approved by worldwide regulatory bodies on a largenumber of products.

During the brazing process, the silver wets and runs over the entiresurface of the dynamic porous coating. This results not only in thedynamic porous coating brazing to the stem, but also provides a thincoating of elemental silver covering the fibers of the SMM spacerfabric. Atomic silver dissolves into silver ions which, at low doses,will eliminate bacterial cells with no toxicological effect on the humanpatient. The surface modification technology applied during theapplication of the dynamic porous coating to the implant is of greatbenefit to effectively preventing deep-seated infection. The silvertreatment enables the steady release of silver ions from the implant'ssurface over several months by dissolution into body fluids, eventuallyleaving a silver-free implant that has long-term durability andbiocompatibility. As patients are at highest risk of infection duringthe initial healing process following surgery, the delayed release ofsilver ions is sufficient to provide a high level of protection.

Coordinating the Attributes of the Dynamic Porous Coating with theAttributes of Human Bone

Human bone is assembled from nano-sized organic and mineral phases intolarge architectures. See FIG. 51. Calcium phosphate crystallites,typically 200-800 angstroms long, 2-5 nm thick, and compositionally andstructurally similar to hydroxyapatite, are typical nanomaterials forforming bone. In addition, other proteins in the extracellular matrix ofbone are nanostructured similar to Type I collagen fibers. The SMMdynamic porous coatings of the present invention can have varying poreand strut sizes, ranging from micro to nano scale, so that the bonecells appropriately interact with the SMM dynamic porous coatings, e.g.,for optimal osseointegration. The SMM dynamic porous coatings of thepresent invention can have engineered nano structures as small as 1 nmin size which can be created to more naturally adhere to the 1 nm bonecrystals, and/or can be engineered so as to be larger, whereby to matchthe larger submicrostructure and microstructure of bone.

Regardless of the manufacturing processes used to create the dynamicporous coating of the present invention, and its pores and struts, theiryield strength and elastic modulus are preferably engineered todesirably match the pore size and stiffness of bone by modifying one ormore of (i) the pore size; (ii) strut thickness, width and length; and(iii) by modifying the amount and position of the packed “buckyball”structures in relation to one another (where a “buckyball” structure isused to form the dynamic porous coating). The SMM dynamic porous coatingcan have an modulus of elasticity of about 5-25 GPa so as to match themodulus of elasticity of cortical bone, or a modulus of elasticity ofabout 1-10 GPa so as to match the modulus of elasticity of cancellousbone. Additionally, the stiffness of the dynamic porous coating can bemodulated so that the proximal region of the dynamic porous coatingmatches the stiffness of the cancellous/trabecular bone and the distalregion of the dynamic porous coating matches the stiffness of thecortical bone. See FIG. 52.

Nitinol

Nitinol is characterized by a specific stress-strain diagram that isdifferent from the deformation behavior of conventional materials, butsimilar to that of living tissues. FIG. 53 presents typicalstress-strain diagrams for stainless steel, NiTi alloy, and livingtissues. In the case of stainless steel, the elastically recoveredstrain (linear portion) is lower than 0.5%. Once the elastic limit isexceeded, stainless steel yields (dislocation slip) and considerableincrease in strain is achieved. This increase in strain, where the metalappears to flow like a viscous liquid, is called plastic deformation andallows the materials to acquire a permanent set that cannot be recoveredafter the stress is released. In shape memory materials such as Nitinol,early deformation is also linearly proportional to the applied stress.Thereafter, deformation continues without a significant increase in theforce (upper loading plateau). During unloading, the constraining forceis again constant over a wide range of shapes (unloading plateau). Up to8% of deformation is recoverable in Nitinol. Bone exhibits more than 1%recoverable strain as well as hysteresis in the loading-unloadingcycles. The similarity in the deformation behavior between Nitinol andliving tissues contributes to the harmonic performance of dynamicimplants under loading-unloading conditions in the body.

The close similarity of Nitinol to natural materials leads to more rapidhealing times, less trauma to surrounding tissue and expeditedosseointegration. When the deforming stress is released, the strain isrecovered at lower stresses.

Method of Shape Setting Shape Memory Alloy (SMA)

Nickel-titanium shape memory metal alloy, Nitinol (NiTi), is afunctional material whose shape and stiffness can be controlled withtemperature. The metal alloy undergoes a complex crystalline-to-solidphase change called martensite-austenite transformation. As the metal inthe high-temperature (austenite) phase is cooled, the crystallinestructure enters the low-temperature (martensite) phase, where it can beeasily bent and shaped. As the metal is reheated above its transitiontemperature, its original shape and stiffness are restored. SMAmaterials exhibit various characteristics depending on the compositionof the alloy and its thermal-mechanical work history. The SMA materialcan exhibit 1-way or 2-way shape memory effects. A 1-way shape memoryeffect results in a substantially irreversible change upon crossing thetransition temperature, whereas a 2-way shape memory effect allows thematerial to repeatedly switch between alternate shapes in response totemperature cycling. SMA can recover large strains in two ways: shapememory effect (SME) and pseudoelasticity, which is also known assuperelasticity (SE). The NiTi family of alloys can withstand largestresses and can recover strains near 8% for low cycle use or up toabout 2.5% strain for high cycle use. The titanium beta and near-betaalloys can have SME and SE.

The shape memory alloys, termed as functional materials, show two uniquecapabilities: shape memory effect (SME) and superelasticity (SE), whichare absent in traditional materials. Both SME and SE largely depend onthe solid-solid, diffusionless phase transformation process known asmartensitic transformation (MT) from a crystallographically more-orderedparent phase (austenite) to a crystallographically less-ordered productphase (martensite). See FIG. 54. The phase transformation (fromaustenite to martensite, or vice versa) is typically marked by fourtransition temperatures, commonly referred to as Martensite finish (Mfor M_(f)), Martensite start (Ms or M_(s)), Austenite finish (Af orA_(f)), and Austenite start (As or A_(s)). If the temperatures forMf<Ms<As<Af, then a change in the temperature within Ms<T<As induces nophase change and both martensite and austenite may coexist withinMf<T<Af. The phase transformations may take place depending on changingtemperature (SME) or changing stress (SE).

Aging of Shape Memory Alloy

In many cases it is desirable for the A_(f) temperature to be close tobody temperature (37° C.). In the case of Nitinol, it is possible tocommercially purchase the starting material with an A_(f) around bodytemperature; however, the transformation temperatures may change as aresult of any cold work and heat treatment steps used to manufacture thefinal product. It is possible to return the Nitinol to its fullyannealed state by heating it to 800° C. to 850° C. for 15 to 60 minutes.This will erase all thermomechanical processing. Following this, theA_(f) temperature can be reset by aging the material. The A_(f)temperature is effected by the exact matrix composition. As can be seenon the Nitinol phase diagram shown in FIG. 55, as the aging temperatureand time increases, nickel rich precipitation reactions occur. Thischanges how much nickel is in the NiTi lattice. By reducing the amountof nickel in the matrix, aging increases the transformation temperature.

It is possible to read a TTT (time-temperature transformation) diagramto determine at what temperature, and for what period of time, to agethe Nitinol material so as to achieve an appropriate A_(f). As seen inthe TTT diagram shown in FIG. 56, aging the Nitinol material at 400° C.for approximately 30 minutes results in an A_(f) close to 37° C. Theexact A_(f) temperature can measured using a differential scanningcalorimeter.

Shape Memory Effect (SME)

For T>Af, the SMA is in the parent austenite phase with a particularsize and shape. Under stress free condition, if the SMA is cooled to anytemperature T<Mf, martensitic transformation (MT) occurs as the materialconverts to product martensite phase. MT is basically a macroscopicdeformation process, though actually no transformation strain isgenerated due to the so-called self-accommodating twinned martensite. Ifa mechanical load is applied to this material and the stress reaches acertain critical value, the pairs of martensite twins begin “detwinning”(conversion) to the stress-preferred twins. The “detwinning” orconversion process is marked by the increasing value of strain withinsignificant increase in stress. The multiple martensite variants beginto convert to a single variant, the preferred variant determined byalignment of the habit planes with the axis of loading. As the singlevariant of martensite is thermodynamically stable at T<As, uponunloading there is no reconversion to multiple variants and only a smallelastic strain is recovered, leaving the materials with a large residualstrain (apparently plastic). Next, if the deformed SMA is heated aboveAf, SMA transforms to parent phase (which has no variants), the residualstrain is fully recovered and the original geometric-configuration isrecovered. It happens as if the material recalls from “memory” itsoriginal shape before the deformation and fully recovers. Therefore,this phenomenon is termed as shape memory effect (1-way SME). However,if some end constraints are used to prevent this free recovery to theoriginal shape, the material generates large tensile recovery stress,which can be exploited as an actuating force for active or passivecontrol purpose. In accordance with the present invention, SMM dynamicporous coatings can be processed via SME.

Superelasticity (SE)

A second feature of SMA is pseudoelasticity, also known assuperelasticity. The superelastic SMA has the unique capability to fullyregain the original shape from a deformed state when the mechanical loadthat causes the deformation is withdrawn. For some superelastic SMAmaterials, the recoverable strains can be on the order of 10%. Thisphenomenon, sometimes termed pseudoelasticity or superelasticity (SE),is dependent on the stress-induced martensitic transformation (SIMT),which in turn depends on the states of temperature and stress of theSMA. To explain the SE effect, consider the case where an SMA, which hasbeen entirely in the parent phase (T>Af), is mechanically loaded.Thermodynamic considerations indicate that there is a critical stress atwhich the crystal phase transformation from austenite to martensite canbe induced. Consequently, the martensite is formed because the appliedstress substitutes for the thermodynamic driving force usually obtainedby cooling for the case of SME. The load, therefore, imparts an overalldeformation to the SMA specimen as soon as a critical stress isexceeded. During unloading, because of the instability of the martensiteat this temperature in the absence of stress, the reverse phasetransformation starts from the SIM to parent phase. When the phasetransformation is complete, the SMA returns to its parent austenitephase. Therefore, superelastic SMA shows a typical hysteresis loop(known as pseudoelasticity or superelasticity), and if the strain duringloading is fully recoverable, it becomes a closed loop. It should benoted that SIMT (or the reverse SIMT) are marked by a reduction of thematerial stiffness. Usually the austenite phase has much higher Young'smodulus in comparison with the martensite phase.

Nitinol cardiovascular stents, orthodontic wires and other commerciallyavailable wire and thin wall tubing products utilize the material'ssuperelastic characteristics. The material's Af temperature is set inrelation to body temperature. Stress induced martensite transformation(SIMT) is used to help collapse the diameter of a device to facilitateminimally invasive insertion into the body. The material is expanded inthe body once free from its constrained/stressed state, desirablyapplying a long-term compression of tissues or bones.

SME and SE Behavior of Dynamic Porous Coating

The SMM dynamic porous coatings of the present invention can exhibit1-way or 2-way shape memory effects and can exhibit SE or SME. See FIG.57.

The dynamic porous coating of the present invention can be designed toexhibit superelastic properties. As an example of a superelastic porouscoating, a Nitinol porous structure is first sintered to the implantstem. Shape setting is then accomplished by firmly constraining thematerial and aging it at approximately 400° C. for 30 minutes, resultingin an A_(f) of 37° C. The time and temperatures can vary depending onthe desired A_(f). The dynamic porous coating is then compressed andconstrained. The implant can then be inserted into the femoral canal andthe constraining force removed. With the constraint removed, the dynamicporous coating will attempt to return to its shape-set geometry as itwarms to 37° C., filling any gaps between the implant and bone, andapplying strain to the bone if the bone keeps the material from fullyrecovering its shape. See FIG. 58. Alternatively, following shapesetting, the implant need not be compressed and constrained. Instead,the implant can be pressed into the femoral canal and deform to fill thespace between the implant and the canal. In this case, the porestructure of the dynamic porous coating can be designed so that thecoating readily collapses.

Alternatively, the dynamic porous coating disclosed in this inventioncan exhibit shape memory effects. As an example, a Nitinol porousstructure is first sintered to the implant stem. The implant and coatingare then aged at approximately 400° C. for 30 minutes, resulting in anA_(f) of 37° C. The time and temperatures can vary depending on thedesired A_(f). The implant is then kept at below its A_(s) temperature,and the dynamic porous coating is compressed. When the compressive forceis removed, the dynamic porous coating will recover approximately 50% ofits strain, leaving the dynamic porous coating with residual strain. Theimplant can then be inserted to the femoral canal, and as it warms to37° C., the dynamic porous coating will expand and recover its originalshape.

When these effects are applied to the honeycomb-shaped structures,similar behavior is observed. See FIG. 59. Additionally, the shaperecovery of the honeycomb, combined with multiple directions of thehoneycomb structure, can be used to force a largely open surface of thedynamic porous coating into direct contact with bone so as to furtherinduce osteointegration and establish early fixation of the implant.

Effect of Dynamic Porous Coating on Osseointegration

Bone adapts and remodels in response to the stress applied to the bone.Wolff's Law states that bones develop a structure which is most suitedto resist the forces acting upon them, adapting both the internalarchitecture and the external conformation to the change in externalloading conditions. When a change in loading pattern occurs, stress andstrain fields in the bone change accordingly. Bone tissue seems to beable to detect the change in strain on a local basis and then adaptaccordingly. The internal architecture is adapted in terms of change indensity, and in disposition, of trabecules and osteons, and the externalconformation in terms of shape and dimensions. When strain isintensified, new bone is formed. On a microscopic scale, bone density israised, and on a macroscopic scale, the bone external dimensions areincremented. When strain is lowered, bone resorption takes place. On amicroscopic scale, bone density is lowered, and on a macroscopic scale,the bone external dimensions are reduced. Undesirable stress shieldingoften causes aseptic implant loosening. When the expanding SMM dynamicporous coating applies stress to the bone tissue, apposition will takeplace and bone density levels will maintain or even increase. Thus, theuse of a dynamic porous coating enhances bone growth adjacent theimplant, which will assist bone ingrowth into the interconnectingdynamic porous coating.

A dynamic porous coating allows surgeons to coat the coating, or fillthe pores of the coating, with hydroxyapatite, tricalcium phosphate andother bone-making agents known in the art, which will remain intactduring implant impaction. Current cementless hip and knee implants, forexample, are wedged into the femoral or tibial bone by means ofhammering the implant with a mallet so as to drive the implant into theprepared bone cavity. A tight interference fit between the implant andfemoral bone, however, may undesirably scrape and/or “squeegee” off anysubstances (e.g., drugs) applied to their surfaces. A dynamic porouscoating will allow the implant to be collapsed through SIMT, coated, andthen inserted into the femoral canal without the “squeegeeing” effect,and thereafter dynamically expanded either through SE or SME. As aresult, the pores of the dynamic porous coating can be treated withbiologically active agents so as to prevent periprosthetic infection.

A dynamic porous coating can be created with two different surfacecharacteristics on the inner and outer surfaces. Using a medullary stentas an example, the outer surface of the stent that comes into contactwith the bone can be prepared with a dynamic porous coating suitable forosseointegration. The inner surface of the stent can be prepared with adynamic porous coating tailored for the generation of new bone marrow.See FIG. 60. The pore size and frequency of the layers of the dynamicporous coating can be varied as required.

Physical Properties of Dynamic Porous Coating

The dynamic porous coating is defined by an open pore structurecomprising of a web of struts that form a basic dodecahedron structurewith major and interconnecting pores. See FIG. 61.

In one preferred form of the invention, the dynamic porous coating hasmajor pore size between approximately 500 μm and 600 μm, andinterconnecting pores between 150 μm and 300 μm. It may be beneficial toinitial fixation and osseointegration for the surface which is in directcontact with the bone to have pores with a small diameter (in the rangeof 200 μm to 500 μm). The average porosity of the dynamic porous coatingis preferably between 65% and 90%. See FIGS. 62-64.

The dynamic porous coating is preferably between approximately 1 mm and5.5 mm in thickness. It may be built out of a single layer, or severallayers stacked on top of each other. See FIG. 65.

Shape Memory Material (SMM) Construct is a Scaffold for Titanium orTantalum Porous Coating

The shape memory material construct described above, which can be madefrom Nitinol wire by weaving, braiding or knitting, honeycomb, ortruss-based structures, sintered beads, and lamination of multiplelayers, can be a dynamic or superelastic scaffold for an additional,more traditional coating to be applied on top of it, e.g., a poroustrabecular analog structure. By way of example but not limitation, theshape memory material construct can be diffusion bonded, sintered, orbrazed to a titanium alloy or cobalt chrome alloy implants to create adynamic or superelastic scaffold. An additional porous coating can bebonded on top of the SMM scaffold construct. This second porous coatingbeing applied on top of the shape memory construct can be a titaniumalloy or tantalum alloy and can be in various porous forms which havealready been commercialized such as Regenerex, Biofoam, Tritanium,Gription, Stiktite, Trabecular Metal, Fiber Metal, etc. On one side,these secondary porous coatings will adhere to the SMM scaffoldconstruct through vapor deposition, chemical vapor deposition, plasmaspray, foam metal, sputtering, sintering powders and beads, and additivemanufacturing processes, and on the other side will be in contact withthe bone tissue. This concept creates a three tier implant:

-   -   Substrate: implant material (titanium or cobalt chrome alloys)    -   Middle Scaffold: Shape Memory Material (Nitinol, exhibiting        superelasticity or shape memory effect)    -   Top Layer: Porous Coating (titanium alloy or tantalum alloy)        this layer contacts the bone.        The SMM scaffold construct can be superelastic (SE) which is        capable of restoring its shape once it is unconstrained and made        to spring back, and/or it can have shape memory effect (SME),        which allows it to be dynamic under the influence of temperature        change, e.g., body temperature. The trabecular coating applied        on top of the SMM substrate can be static or dynamic. However,        it is the bottom SMM scaffold construct that will give the top        layer elasticity, flexibility and superelasticity.

Dynamic Porous Coating

Thus it will be seen that the present invention comprises the provisionand use of a novel dynamic porous coating, preferably made of shapememory material, that is applied and bonded to the surface of anorthopedic implant and which is capable of expanding once inserted intothe bone via superelasticity or shape memory effect (temperature change)so as to create an interference fit between the implant and bone tissue.The expansion may be initiated by either removal of a containment sleeve(superelasticity effect), or by the warming of the material from atemperature below body temperature to body temperature (shape memoryeffect). The strain from the expanding implant can cause boneremodeling, enhance osseointegration and will facilitate immediatefixation and long term apposition. The structure of the dynamic porouscoating can be 3D with interconnection pores similar to trabecular boneor a 3D honeycomb structure to facilitate bone ingrowth into thecoating. The strength and stiffness of the dynamic porous coating can beaccurately matched to bone.

Modifications of the Preferred Embodiments

It should be understood that many additional changes in the details,materials, steps and arrangements of parts, which have been hereindescribed and illustrated in order to explain the nature of the presentinvention, may be made by those skilled in the art while still remainingwithin the principles and scope of the invention.

What is claimed is:
 1. A dynamic porous coating for an orthopedicimplant, wherein the dynamic porous coating is adapted to apply anexpansive force against adjacent bone so as to fill gaps between thedynamic porous coating and adjacent bone and to create an interferencefit between the orthopedic implant and the adjacent bone.
 2. A dynamicporous coating according to claim 1 wherein the dynamic porous coatingcomprises shape memory material.
 3. A dynamic porous coating accordingto claim 2 wherein the expansive force is created by the superelasticityof the shape memory material.
 4. A dynamic porous coating according toclaim 2 wherein the expansive force is created by the shape memoryeffect of the shape memory material.
 5. A dynamic porous coatingaccording to claim 1 wherein the dynamic porous coating is elastic.
 6. Adynamic porous coating according to claim 5 wherein the dynamic porouscoating comprises a shape memory material, and further wherein theexpansive force is created by the superelasticity of the shape memorymaterial.
 7. A dynamic porous coating according to claim 1 wherein thedynamic porous coating comprises spacer fabric.
 8. A dynamic porouscoating according to claim 7 wherein the spacer fabric comprisesfilaments formed out of shape memory material.
 9. A dynamic porouscoating according to claim 8 wherein the expansive force is created bythe superelasticity of the shape memory material.
 10. A dynamic porouscoating according to claim 8 wherein the expansive force is created bythe shape memory effect of the shape memory material.
 11. A dynamicporous coating according to claim 1 wherein the expansive force appliedby the porous coating causes bone remodeling.
 12. A dynamic porouscoating according to claim 1 wherein the dynamic porous coating isformed by coating foam with a shape memory material, removing the foamand then attaching the resulting structure to the orthopedic implant.13. A dynamic porous coating according to claim 1 wherein the dynamicporous coating comprises wire formed out of a shape memory material andwhich is knit, woven or braided.
 14. A dynamic porous coating accordingto claim 1 wherein the dynamic porous coating comprises a honeycomb ortruss-based structure formed out of shape memory material.
 15. A dynamicporous coating according to claim 1 wherein the dynamic porous coatingcomprises sintered beads formed out of shape memory material.
 16. Adynamic porous coating according to claim 1 wherein the dynamic porouscoating comprises multiple layers laminated to one another, and furtherwherein at least one of the multiple layers comprises shape memorymaterial.
 17. An orthopedic implant comprising a substrate and a dynamicporous coating secured to the substrate, wherein the dynamic porouscoating is adapted to apply an expansive force against adjacent bone soas to fill gaps between the dynamic porous coating and adjacent bone andto create an interference fit between the orthopedic implant and theadjacent bone.
 18. An orthopedic implant according to claim 17 whereinthe dynamic porous coating comprises shape memory material.
 19. Anorthopedic implant according to claim 18 wherein the expansive force iscreated by the superelasticity of the shape memory material.
 20. Anorthopedic implant according to claim 18 wherein the expansive force iscreated by the shape memory effect of the shape memory material.
 21. Anorthopedic implant according to claim 17 wherein the dynamic porouscoating is elastic.
 22. An orthopedic implant according to claim 21wherein the dynamic porous coating comprises a shape memory material,and further wherein the expansive force is created by thesuperelasticity of the shape memory material.
 23. An orthopedic implantaccording to claim 17 wherein the dynamic porous coating comprisesspacer fabric.
 24. An orthopedic implant according to claim 23 whereinthe spacer fabric comprises filaments formed out of shape memorymaterial.
 25. An orthopedic implant according to claim 24 wherein theexpansive force is created by the superelasticity of the shape memorymaterial.
 26. An orthopedic implant according to claim 24 wherein theexpansive force is created by the shape memory effect of the shapememory material.
 27. An orthopedic implant according to claim 17 whereinthe expansive force applied by the porous coating causes boneremodeling.
 28. An orthopedic implant according to claim 17 wherein thedynamic porous coating is formed by coating foam with a shape memorymaterial, removing the foam and then attaching the resulting structureto the orthopedic implant.
 29. An orthopedic implant according to claim17 wherein the dynamic porous coating comprises wire formed out of ashape memory material and which is knit, woven or braided.
 30. Anorthopedic implant according to claim 17 wherein the dynamic porouscoating comprises a honeycomb or truss-based structure formed out ofshape memory material.
 31. An orthopedic implant according to claim 17wherein the dynamic porous coating comprises sintered beads formed outof shape memory material.
 32. An orthopedic implant according to claim17 wherein the dynamic porous coating comprises multiple layerslaminated to one another, and further wherein at least one of themultiple layers comprises shape memory material.
 33. A method forproviding therapy to a patient, the method comprising: providing anorthopedic implant comprising a substrate and a dynamic porous coatingsecured to the substrate, wherein the dynamic porous coating is adaptedto apply an expansive force against adjacent bone so as to fill gapsbetween the dynamic porous coating and adjacent bone and to create aninterference fit between the orthopedic implant and the adjacent bone;inserting the orthopedic implant into a bone cavity in the patient sothat the dynamic porous coating applies an outward force againstadjacent bone so as to fill gaps between the dynamic porous coating andadjacent bone and to create an interference fit between the orthopedicimplant and the adjacent bone.
 34. A method according to claim 33wherein the dynamic porous coating comprises shape memory material. 35.A method according to claim 34 wherein the expansive force is created bythe superelasticity of the shape memory material.
 36. A method accordingto claim 34 wherein the expansive force is created by the shape memoryeffect of the shape memory material.
 37. A method according to claim 33wherein the dynamic porous coating is elastic.
 38. A method according toclaim 37 wherein the dynamic porous coating comprises a shape memorymaterial, and further wherein the expansive force is created by thesuperelasticity of the shape memory material.
 39. An orthopedic implantaccording to claim 33 wherein the dynamic porous coating comprisesspacer fabric.
 40. A method according to claim 39 wherein the spacerfabric comprises filaments formed out of shape memory material.
 41. Amethod according to claim 40 wherein the expansive force is created bythe superelasticity of the shape memory material.
 42. A method accordingto claim 40 wherein the expansive force is created by the shape memoryeffect of the shape memory material.
 43. A method according to claim 33wherein the expansive force applied by the porous coating causes boneremodeling.
 44. A method according to claim 33 wherein the dynamicporous coating is formed by coating foam with a shape memory material,removing the foam and then attaching the resulting structure to theorthopedic implant.
 45. A method according to claim 33 wherein thedynamic porous coating comprises wire formed out of a shape memorymaterial and which is knit, woven or braided.
 46. A method according toclaim 33 wherein the dynamic porous coating comprises a honeycomb ortruss-based structure formed out of shape memory material.
 47. A methodaccording to claim 33 wherein the dynamic porous coating comprisessintered beads formed out of shape memory material.
 48. A methodaccording to claim 33 wherein the dynamic porous coating comprisesmultiple layers laminated to one another, and further wherein at leastone of the multiple layers comprises shape memory material.
 49. Adynamic porous coating for an orthopedic implant, the dynamic porouscoating comprising: a spacer fabric comprising: an inner layer formed byfibers; an outer layer formed by fibers; the outer layer being spacedfrom the inner layer; and the outer layer being connected to the innerlayer by a plurality of connecting fibers extending between the innerlayer and the outer layer, such that the outer layer is capable ofapplying an outward force against adjacent bone.
 50. A dynamic porouscoating according to claim 49 wherein the dynamic porous coating iselastic.
 51. A dynamic porous coating according to claim 50 wherein theconnecting fibers are formed from a superelastic material, and furtherwherein the outward force is generated by superelasticity.
 52. A dynamicporous coating according to claim 50 wherein the connecting fibers areformed from a superelastic material, and further wherein the outwardforce is generated by the shape memory effect.
 53. A dynamic porouscoating according to claim 50 wherein the space between the inner layerand the outer layer is characterized by a plurality of channels formedby the connecting fibers.
 54. A dynamic porous coating according toclaim 50 wherein the spacer fabric has a thickness of from about 0.25 mmto about 10 mm.
 55. A dynamic porous coating according to claim 50wherein the spacer fabric is approximately 50-98% porous.
 56. A dynamicporous coating according to claim 50 wherein the spacer fabric comprisesa bioresorbable material.
 57. A dynamic porous coating according toclaim 50 wherein the fibers of the inner layer and the outer layer, andthe connecting fibers, comprise at least two different types of fibers.58. A dynamic porous coating according to claim 50 wherein the spacerfabric is configured to have a modulus of elasticity of approximately0.100-20 GPa.
 59. A dynamic porous coating according to claim 50 whereinthe spacer fabric has approximately 3-99% strain recovery.
 60. A dynamicporous coating according to claim 50 wherein the spacer fabric iscollapsed and maintained in a collapsed condition using a resorbablecalcium phosphate bone putty or cement which, upon resorbing, allows thespacer fabric to resume its previous state.
 61. A dynamic porous coatingaccording to claim 50 wherein the spacer fabric is secured to anorthopedic implant by one of brazing with silver, sintering, anddiffusion bonding.
 62. An orthopedic implant according to claim 17wherein the orthopedic implant comprises one from the group consistingof a femoral hip stem implant, an acetabular hip implant, a femoralcondyle knee implant, a tibial tray knee implant, a shoulder implant,and an elbow implant.
 63. A method according to claim 33 wherein theorthopedic implant is inserted into the patient during revision surgery.64. A dynamic porous coating according to claim 49 wherein the innerlayer comprises pores, the outer layer comprises pores, and the size ofthe pores of the inner layer differs from the size of the pores of theouter layer.
 65. A dynamic porous coating according to claim 49 whereinat least some of the fibers are coated.
 66. An orthopedic implantaccording to claim 17 wherein the orthopedic implant comprises one fromthe group consisting of a spinal implant, an extremity implant, a dentalimplant, a cranial implant, and a maxillofacial implant.
 67. A dynamicporous coating according to claim 1 wherein the spacer fabric hasapproximately 3-99% strain recovery.
 68. An orthopedic implant accordingto claim 17 wherein the spacer fabric has approximately 3-99% strainrecovery.
 69. A dynamic porous coating according to claim 49 wherein thespacer fabric has a modulus of elasticity which varies according to thedegree of compression imposed on the spacer fabric.
 70. A dynamic porouscoating according to claim 69 wherein the spacer fabric has a relativelylow modulus of elasticity when the spacer fabric has relatively littlecompression imposed on it, and further wherein the spacer fabric has arelatively high modulus of elasticity when the spacer fabric has a largecompression imposed on it.
 71. A dynamic porous coating according toclaim 1 wherein the dynamic porous coating is capable of expanding atleast 0.025 inch.
 72. An orthopedic implant according to claim 17wherein the dynamic porous coating is capable of expanding at least0.025 inch.
 73. A dynamic porous coating according to claim 49 whereinthe dynamic porous coating is capable of expanding at least 0.025 inch.